You will not find a Pirate Boss on small sailboats. The pirates spawn either on the deck or riding in small boats. You will find these below-decks or in the cabin, depending on the ship's layout. They always contain one or two chests, but you may find Furnaces, Crafting Tables, Jukeboxes, and Beds as well. The shape, size, and sail design varies from ship to ship. Most of the pirate ships you'll find are rather small and contain 1-5 spawners.
